U.S. patent number 5,095,491 [Application Number 07/685,306] was granted by the patent office on 1992-03-10 for laser system and method.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to William J. Kozlovsky, William P. Risk.
United States Patent |
5,095,491 |
Kozlovsky , et al. |
March 10, 1992 |
Laser system and method
Abstract
A laser system uses a nonlinear ring resonator to produce
Type-II nonlinear second harmonic generated light. The reflective
surfaces of the resonator are all symmetrical with respect to the
crystal axes. This allows orthogonally polarized light to be
resonated along the same beampath in the resonator without
experiencing bireflection.
Inventors: |
Kozlovsky; William J. (Mountain
View, CA), Risk; William P. (Mountain View, CA) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
24751612 |
Appl.
No.: |
07/685,306 |
Filed: |
April 12, 1991 |
Current U.S.
Class: |
372/94;
G9B/7.102; 372/27; 372/21; 372/108 |
Current CPC
Class: |
G02F
1/37 (20130101); H01S 5/0687 (20130101); G02F
1/3542 (20210101); G02F 1/3509 (20210101); H01S
5/0683 (20130101) |
Current International
Class: |
G02F
1/37 (20060101); G02F 1/35 (20060101); G11B
7/125 (20060101); H01S 5/0683 (20060101); H01S
5/00 (20060101); H01S 5/0687 (20060101); H01S
003/083 () |
Field of
Search: |
;372/94,108,21,27,92,98,75,60 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4791631 |
December 1988 |
Baumert et al. |
5048047 |
September 1991 |
Kozlovsky et al. |
|
Other References
L Goldberg and M. K. Chun, "Applied Physics Letters", vol. 55, p.
218, Jul. 17, 1979. .
P. Gunter, et al., "Applied Physics Letters", vol. 35, p. 461, Sep.
15, 1979. .
A. Ashkin, et al., "IEEE Journal of Quantum Electronics", vol.
QE-2, p. 109, Jun. 1966. .
W. J. Kozlovsky, et al., "IEEE Journal of Quantum Electronics",
vol. 24, p. 731, Jun. 1988. .
J. C. Baumert, et al., "Applied Physics Letters", vol. 51, p. 2192,
Dec. 1987..
|
Primary Examiner: Scott, Jr.; Leon
Attorney, Agent or Firm: Millett; Douglas R.
Claims
What is claimed is:
1. A laser system comprising:
a first radiation source for producing radiation of a first
polarization;
a second radiation source for producing radiation of a second
polarization different from said first polarization;
a nonlinear crystal for receiving the first and second polarization
radiation and producing second harmonic radiation; and
a ring resonator integral to the nonlinear crystal for resonating
the first and second polarization radiation, the resonator having a
plurality of reflective surfaces which are symmetrical with respect
to the axes of the nonlinear crystal, whereby the first and second
polarization radiation is resonated without experiencing
bireflection.
2. The system of claim 1, wherein the first and second radiation
sources are diode lasers.
3. The system of claim 1, wherein the nonlinear crystal is a
Type-II nonlinear crystal.
4. The system of claim 1, wherein the reflective surfaces form a
paralellogram shaped resonator path.
5. A method for frequency conversion of radiation comprising the
steps of:
generating a first radiation beam of a first polarization;
generating a second radiation beam of a second polarization
different from said first polarization;
coupling the first and second polarization radiation beams to a
nonlinear crystal; and
resonating the first and second polarization radiation beams inside
the nonlinear crystal by reflecting the beams off of a plurality of
reflective surfaces which are symmetrical with respect to the axes
of the nonlinear crystal to produce second harmonic generated
radiation, whereby the first and second polarization radiation
beams are resonated without experiencing bireflection.
6. The method of claim 5 wherein, the radiation beams are produced
by laser diodes.
7. The method of claim 5 wherein, the nonlinear crystal is a
Type-II nonlinear crystal.
8. The method of claim 5, wherein the reflective surfaces form a
parallelogram shaped resonator path.
9. A laser system comprising:
a first laser for producing a radiation beam of a first
polarization;
a second laser for producing a radiation beam of a second
polarization different from said first polarization;
a nonlinear crystal for receiving the first and second polarization
radiation and for producing second harmonic radiation; and
a resonator for resonating the first and second polarization
radiation, having four reflective surfaces integral to the
nonlinear crystal, each of the reflective surfaces being
symmetrical with respect to the axes of the nonlinear crystal,
whereby the first and second polarization radiation is resonated
without experiencing bireflection.
10. The system of claim 9, wherein the first and second radiation
beams are 995-1000 nm in wavelength.
11. The system of claim 9, wherein the first radiation beam is
810-820 nm in wavelength and the second radiation beam is 1040-1070
nm in wavelength.
12. The system of claim 9, wherein the nonlinear crystal is
KTP.
13. The system of claim 9, wherein the reflective surfaces are
arranged such that the first and second polarization beams travel
along a parallelogram shape beam path.
14. A laser system comprising:
a first radiation source for producing radiation of a first
polarization;
a second radiation source for producing radiation of a second
polarization different from said first polarization;
a nonlinear crystal for receiving the first and second polarization
radiation and for producing second harmonic radiation;
a ring resonator arranged around the nonlinear crystal for
resonating the first and second polarization radiation, the
resonator having a plurality of reflective surfaces which are
symmetrical with respect to the axes of the nonlinear crystal,
whereby the first and second polarization radiation is resonated
without experiencing bireflection;
an optical data storage medium;
an optical transmission means for directing said second harmonic
radiation from the resonator to the optical data storage medium;
and
an optical reception means for receiving a reflected second
harmonic radiation beam from the medium and for providing a data
signal responsive thereto.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to laser systems and more particularly to
laser systems which produce light by frequency doubling or
sum-frequency mixing.
2. Description of the Prior Art
Semiconductor diode lasers are of interest for a number of
applications such as optical data storage, laser printing, and
biochemical analysis. One example is the gallium-aluminum-arsenide
(GaAlAs) diode laser which generates laser light in the
near-infrared range (750-860 nm in wavelength). In optical data
storage systems, the light from the laser diode is focused onto a
spot on the optical disk in order to record each bit of data. The
spot size is equal to approximately .lambda./(2*(N.A.)), where
.lambda. is the wavelength of the light and (N.A.) is the numerical
aperture of the focusing lens. In typical systems, the (N.A.) is
approximately 0.5 and the resulting spot size is approximately 800
nm in diameter.
It is apparent that if the wavelength of the laser light can be cut
in half, the diameter of the spot size will also be cut in half and
the overall storage density on the optical disk may be quadrupled.
Unfortunately, laser diodes that produce light in the blue
wavelength range (430 nm in wavelength) are not available.
One technique to convert light to a higher frequency (shorter
wavelength) is known as second harmonic generation (SHG). A laser
beam at a first lower frequency is passed through a nonlinear
crystal, such as potassium niobate (KNbO.sub.3), which produces a
second harmonic laser beam (i.e., a beam at twice the frequency of
the original laser beam which entered the nonlinear crystal). This
SHG technique is discussed in articles by M. K. Chun, et al.,
Applied Physics Letters, Vol. 55, p. 218, July 17, 1989; and P.
Gunter, et al., Applied Physics Letters, Vol. 35, p. 461, Sept. 15,
1979.
However, since the diode laser's output power is low, techniques to
improve the second harmonic generation efficiency are required in
order to produce a useful and efficient laser system.
One way to increase the efficiency of the SHG scheme is to place an
optical resonator or cavity around the nonlinear crystal. The light
is reflected back and forth through the crystal inside the
resonator in order to generate a substantial amount of the blue
light. This technique was originally proposed and demonstrated by
Ashkin, et al., IEEE Journal of Quantum Electronics, Vol. QE-2, p.
109, 1966. Other examples include Goldberg, et al., Applied Physics
Letters, Vol. 55, p. 218, 1989; and Baer, et al., Conference on
Lasers and Electro-Optics, Paper THM5, 1989. Frequency doubling of
GaAlAs diode lasers using a build-up cavity containing a nonlinear
crystal such as potassium niobate (KNbO.sub.3) offers the potential
for the design of simple, compact laser systems. For the build up
to occur, the external cavity resonance frequency must match the
diode laser frequency, and the prior art includes a variety of
techniques for achieving this frequency matching (e.g., Dixon, et
al., Optics Letters, Vol. 14, p. 731, 1989; R. W. Drever, et al.,
Applied Physics B, Vol. 31, p. 97, 1983; and W. J. Kozlovsky, et
al., IEEE Journal of Quantum Electronics, Vol. 24, p. 913,
1988.)
Heretofore, the nonlinear crystal KNbO.sub.3 has been used for
resonantly enhanced frequency doubling of GaAlAs laser diodes.
Potassium niobate has a large nonlinear coefficient and sufficient
birefringence for phasematching of second-harmonic generation at
the wavelengths of GaAlAs laser diodes. However, this phasematching
is very sensitive to the temperature of the crystal, and this
temperature must be precisely controlled to maintain efficient
second-harmonic generation.
Nonlinear crystals other than KNbO.sub.3 have been shown to have
phasematching properties advantageous for generation of blue/green
light by frequency upconversion of semiconductor laser diodes. In
particular, potassium titanyl phosphate (KTiOPO.sub.4, KTP) can be
used for second-harmonic generation of 990 nm strained-layer InGaAs
laser diodes (e.g., W. P. Risk, et al. Applied Physics Letters,
Vol. 55, No. 12, p. 1179, and pending U.S. patent application Ser.
No. 07/570,251 filed Aug. 17, 1990 by Harder, et al.) and has been
shown to have broad temperature tolerances in that application.
Similarly, sum-frequency mixing in KTP (e.g., J. C. Baumert, et al.
Applied Physics Letters, Vol. 51, p. 2192, 1987 and U.S. Pat. No.
4,791,631) of a wavelength <994 nm with a wavelength >994 nm
supplied by a combination of GaAlAs and InGaAs lasers can be used
to generate virtually any blue/green wavelength between 450 nm and
500 nm.
The nonlinear processes described above in KTP require the presence
of two orthogonally polarized infrared lightwaves in order to be
efficient. Such interactions are known as Type-II nonlinear
interactions. This is in contrast to the case of second-harmonic
generation in KNbO.sub.3 where only one polarization is required
(Type-I nonlinear interaction). Hence, to enhance the efficiency of
a Type-II nonlinear interaction requires that two lightwaves having
orthogonal polarizations, at the same or different wavelengths, be
simultaneously resonated.
Monolithic resonators have reflective surfaces which are integrally
formed on the nonlinear crystals such that the resonating
lightwaves never leave the nonlinear crystal. This is highly
desirable for reasons of efficiency, stability, and compactness.
The preferred configuration in a Type-I nonlinear process is a
triangular ring resonator which has a three sided beampath.
The inventors have discovered that enhancement of a Type-II
nonlinear process has never previously been demonstrated in a
monolithic ring resonator, due to the phenomenon of bireflection.
Bireflection causes the ring path to depend on polarization of the
light wave, so that the two polarizations required for a Type-II
interaction cannot be simultaneously resonated in a triangular
three-mirror monolithic ring, such as that used for Type-I
interactions. What is needed is a new ring resonator geometry that
permits both of the waves required for the Type-II interaction to
be simultaneously resonated along the same ring path.
SUMMARY OF THE INVENTION
In the preferred embodiment of the present invention, a monolithic
ring resonator consisting of four reflecting surfaces polished
directly onto the KTP crystal is designed to create a
rhomboid-shaped ring path within the KTP crystal. This rhomboidal
ring path is designed to be geometrically symmetric about an axis
of crystal symmetry for the KTP crystal. This arrangement defines a
closed ring path independent of the polarization or wavelength of
the light.
Light from each of two diode lasers is passed through a collimating
lens, a circularizing prism, a Faraday isolator, and a focusing
lens. The focusing lens directs the light into the nonlinear
crystal. The angle of incidence for the two beams is different,
depending on the wavelength and polarization of the light. The two
lasers are locked to resonant frequencies of the cavity by one of
the techniques referenced above as part of the prior art.
Inside the nonlinear crystal, both input beams follow the same
rhomboidal path through the crystal and build-up to high powers due
to resonant enhancement. Blue/green light is generated through the
nonlinear interaction and is transmitted out of the monolithic KTP
crystal.
For a fuller understanding of the nature and advantages of the
present invention, reference should be made to the following
detailed description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of the rhomboidal-path monolithic
ring resonator of the present invention;
FIG. 2 shows a schematic diagram of a laser system for frequency
upconversion of infrared laser diodes using the resonator of FIG.
1;
FIG. 3A is a slowness curve which describes the propagation of
light in an anisotropic optical crystal;
FIG. 3B is a slowness curve which shows the problem of bireflection
which prevents a triangular ring path from being used in the
resonator of FIG. 1;
FIG. 3C is a slowness curve which shows how bireflection is
eliminated using mirrors parallel to planes of crystal
symmetry;
FIG. 4 shows a schematic diagram of a phase-change storage system
of the present invention;
FIG. 5 shows a schematic diagram of a magneto-optic storage system
of the present invention
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a nonlinear resonator of the present invention and is
designated by the general reference number 10. Nonlinear resonator
10 is fabricated from a nonlinear crystal material 12, such as
potassium titanyl phosphate (KTP). The specific material 12 is
polished to have two curved mirrors 14 and 16 at both ends and flat
total-internal-reflection surfaces 18 and 20 parallel to each other
and to the mirror axes. Mirrors 14 and 16 are formed of dielectric
coatings deposited directly on the polished ends of the crystal.
The orientation of the nonlinear crystal 12 is chosen so a mirror
axis 21 lies along an axis of crystal symmetry, to eliminate
bireflection and to permit phasematching using the maximum
birefringence available in the crystal. Nonlinear crystals have
crystallographic axes a, b and c as are known in the art. In the
preferred embodiment using KTP as the nonlinear crystal 12, the
mirror axis 21 is chosen to lie along the crystallographic b-axis
and the surfaces 18 and 20 are perpendicular to the a-axis.
The spacings of the curved mirrors 14 and 16 and the flat
total-internal-reflection mirrors 18 and 20 are chosen to provide a
closed four sided stable mode ring path 22 within the crystal. Ring
path 22 describes the path of propagation within the nonlinear
crystal 20 of two resonator modes 26 and 28. Resonator mode 26
comprises light having polarization perpendicular to the plane of
ring path 22; resonator mode 28 comprises light having polarization
parallel to the plane of ring path 22. The spatial distribution of
optical energy in these two modes may be slightly different due to
differences in wavelength and refractive index; however the energy
in both modes occupies substantially the same volume (i.e., they
travel the same ring path). In addition, the frequencies at which
the nonlinear resonator 10 is resonant and causes intensity
build-up in the modes 26 and 28 is different for the two modes due
to differences in the refractive index. The radii of curvature of
the curved mirrors 14 and 16 are preferrably chosen to produce a
small beam waist (20 microns at 1/e.sup.2 beam radius, i.e., the
radius of the beam where power equals 1/e.sup.2 of peak power) for
the resonant modes 26 and 28. In a specific embodiment, the radii
of curvature of mirrors 14 and 16 are approximately 5 cm, the
spacing between mirrors 14 and 16 is approximately 6 mm and the
spacing between flat mirrors 18 and 20 is approximately 1 mm. It is
desirable for an opening angle 24 of the rhomboidal ring path to be
small for phasematching of the nonlinear interaction to yield the
shortest possible second harmonic (blue/green) wavelength and for
the astigmatism to be minimized. Path 22 should be as close as
possible to being parallel to the b-axis. The thickness of the
crystal 12 is the limiting factor. In the preferred embodiment, the
opening angle is approximately 9.1.degree. and the phasematching
wavelength is 996 nm.
The nonlinear resonator 10 is aligned with an orientation to
receive optically incident input fundamental frequency beams 30 and
32. Beam 30 having its polarization perpendicular to the plane of
ring path 22 and beam 32 having its polarization parallel to the
ring path 22. The orientations of the polarizations of beams 30 and
32 are represented as a dot and arrow, respectively in FIG. 1. In a
preferred embodiment, beams 30 and 32 are approximately 996 nm in
wavelength and enter resonator 10 at an angle of 16.9.degree. and
16.0.degree. , respectively, relative to the axis 21. The diameters
of beams 30 and 32 at the input mirror 14 are chosen by spatial
mode matching considerations for most efficient excitation of
fundamental resonator modes 26 and 28. Spatial mode matching is
discussed in more detail in the article, "Laser Beams and
Resonators," Applied Optics, Vol. 5, p. 1550, (1966).
The dielectric mirror coatings 14 and 16 are designed to maximize
the build-up of intensity in resonator modes 26 and 28 when excited
by input beams 30 and 32. The mirror 16 is chosen to be highly
reflective at the wavelengths corresponding to both modes 26 and 28
and highly transmissive at the wavelength of the second harmonic
(blue/green) output beams 34 and 36. The reflectivity of the input
mirror 14 is chosen for optimum coupling of the input beams 30 and
32 to the cavity modes 26 and 28, taking into account both the
internal losses of the nonlinear crystal, i.e. round trip loss of
approximately 0.5% in a specific embodiment of a 6 mm crystal
length, and the anticipated losses due to the nonlinear
interaction. This optimum coupling is known as impedance matching
and is described in the article "Efficient Second Harmonic
Generation of a Diode-Laser-Pumped Nd:YAG Laser," by W.J.
Kozlovsky, et al., IEEE J. Quantum Electronics, Vol. QE-24, No. 6,
p. 913, (1988).
In a preferred embodiment, with 80% spatial mode matching of 50
milliwatts input beams mirror 14 has a reflectivity of 98.9% for
the fundamental frequency of beams 30 and 32, and a reflectivity of
less than 20% for the frequency of the second harmonic output beams
34 and 36. Mirror 16 has a high reflectivity approximately 99.9%
for the frequency of beams 30 and 32, and a reflectivity of less
than 20% for the frequency of the second harmonic output beams 34
and 36.
If each incident beam 30 and 32 is tuned to a resonant frequency of
its corresponding resonator mode 26 and 28, respectively, the
optical power in modes 26 and 28 will build up to a high intensity.
In addition, the frequencies f.sub.1 and f.sub.2 of resonant modes
26 and 28 and the frequency f.sub.3 =f.sub.1 +f.sub.2 of the
generated beams 34 and 36 must satisfy the phasematching condition
for the nonlinear interaction, f.sub.1 n.sub.1 +f.sub.2 n.sub.2
=f.sub.3 n.sub.3, where f.sub.1 is the frequency of mode 26,
f.sub.2 is the frequency of mode 28, f.sub.3 is the frequency of
output beams 34 and 36, n.sub.1 is the refractive index of the
crystal 12 for mode 26, n.sub.2 is the refractive index of the
crystal 12 for mode 28, and n.sub.3 is the refactive index of the
crystal 12 for output beams 34 and 36.
The operation of resonator 10 may now be understood. Fundamental
frequency beams 30 and 32 are coupled into resonator 10 at mirror
14. A small portion of beams 30 and 32 do not enter resonator 10
and are reflected as beams 40 and 42, respectively. Beams 30 and 32
resonate inside crystal 12 along beampath 22. Beams 30 and 32
travel along the same beampath 22 because they experience the same
angle of reflection at each reflective surface. Note that
reflective surfaces 18 and 20 are parallel to the b and c-axis, and
that the tangential planes at the reflective points at mirrors 14
and 16 are parallel to the a and c-axis. It is this symmetry that
eliminates the bireflection problem and allows the beams to
resonate along the same beampath. A more detailed description of
this bireflection problem is given below.
The beams 30 and 32 interact with crystal 12 to achieve a Type-II
nonlinear interaction which results in the production of second
harmonic (SH) light. This SH light exits mirror 14 as beam 34 and
exits mirror 16 as beam 36.
Alternative embodiments of resonator 10 are possible. For example,
the beams 30 and 32 may have different frequencies. Also, mirror 14
may be designed to resonate the second harmonic ligt rather than
let it escape as beam 34. In such a case, only beam 36 will be
output.
FIG. 2 shows a laser system 100 using the nonlinear resonator 10. A
laser diode 110 generates a laser beam 30 which is collimated by a
lens 112 and is circularized by a circularizing prism 114. The
light 30 is then passed through a Faraday isolator 116. Faraday
isolator 116 is used to prevent back-reflected light and scattered
light from reaching diode laser 110. A lens 118 couples the light
30 into the fundamental spatial mode 26. A second laser diode 120
generates a laser beam 32 which is collimated by a lens 122 and is
circularized by a circularizing prism 124. The light 32 is then
passed through a Faraday isolator 126. Faraday isolator 126 is used
to prevent back-reflected light and scattered light from reaching
diode laser 120. A lens 128 couples the light 32 into the
fundamental spatial mode 28.
In a preferred embodiment, laser diodes 110 and 120 are both
strained-layer InGaAs/GaAs laser diodes operating at wavelengths of
approximately 995-1000 nm. In this embodiment, the wavelength of
output beams 34 and 36 are approximately 499 nm. In a another
specific embodiment, laser diode 110 is a GaAlAs laser diode
operating at a wavelength of 810-820 nm and laser diode 120 is a
strained-layer InGaAs laser diode operating at a wavelength of
1040-1070 nm. In this embodiment, the wavelength of the output
beams 34 and 36 are approximately 462 nm.
The frequencies of laser diodes 110 and 120 must be maintained at
resonant frequencies of modes 26 and 28. In a preferred embodiment,
this is done by sensing the beams 40 and 42 reflected from the
cavity with optical detectors 134 and 136, respectively. A pair of
locking electronic circuits 138 and 140 are connected to detectors
134 and 136, respectively. Circuits 138 and 140 are connected to
and control lasers 110 and 120, respectively. Reflected beams 40
and 42 contain FM signals. The FM signals contained in reflected
beams 40 and 42 can be used by circuits 138 and 140 to adjust the
frequencies of laser diodes 110 and 120 to match the cavity
resonance frequencies. This FM locking techniques is taught in W.
J. Kozlovsky, et al., Applied Physics Letters, Vol. 56, p. 2291,
(1990). Other frequency locking techniques may also be used.
FIGS. 3A, 3B and 3C are slowness curves which explain why
orthogonally polarized modes 26 and 28 can have the same geometric
ring path 22 in the nonlinear resonator 10 and why the unique
geometry of this invention is required. FIG. 3A shows a slowness
curve for the nonlinear material KTP. The curves 200 and 202 depict
the refractive index characteristic of a light wave propagating at
an angle 204 with respect to the a-axis of the KTP crystal; that
is, the curves indicate the speed at which a light wave of a given
polarization will travel in a particular direction within the
crystal. Curve 200 applies to a lightwave polarized parallel to the
a-b plane; curve 202 applies to a lightwave polarized perpendicular
to the a-b plane. If the light wave is polarized perpendicular to
the a-b plane, it will have refractive index n.sub.1 as given by
the length of arrow 206 (the distance from origin to curve 202). If
the light wave is polarized parallel to the a-b plane, it will have
refractive index n.sub.2 as given by the length of arrow 208 (the
distance from the origin to curve 200). As a specific example, if
angle 204 is 90.degree., the light waves are propagating along the
b-axis of the crystal. The light wave polarized parallel to the a-b
plane will have index of refraction n.sub.a and the light wave
polarized perpendicular to the a-b plane will have index of
refraction n.sub.c
FIG. 3B shows the case typical of a three-mirror ring resonator as
is commonly used for Type-I frequency doubling of GaAlAs lasers
with the nonlinear material KNbO.sub.3. A wave with polarization
perpendicular to the a-b plane is incident upon reflecting surface
314, and is represented by an arrow 310 oriented in the direction
of propagation and with length (the distance from curve 202 to the
origin) equal to the refractive index of the wave, in this case,
n.sub.c Reflecting surface 314 is oriented at an angle to the
crystallographic a-and b-axes. Electromagnetic boundary conditions
require that the reflected wave travel in such a direction that
that its effective speed along the direction of the reflecting
surface be the same as that of the incident wave. The effective
speed of the wave along the direction of the reflecting surface is
represented by the projection of arrow 310 onto the reflecting
surface 314. Therefore, for the wave represented by arrow 310, this
condition is satisfied by a reflected wave propagating in the
direction of arrow 316. The length of arrow 316 is from the origin
to curve 202. A second incident wave, represented by arrow 312, is
polarized parallel to the a-b plane and is incident upon reflecting
surface 314. The length of arrow 312 is from curve 200 to the
origin. In this case, the wave reflected from the boundary must
travel in the direction of arrow 318 in order for the effective
speed along the direction of the reflecting surface 314 to be the
same as that of the incident wave. The length of arrow 318 is from
the origin to curve 200. In this case, it can be seen that even
though both waves are incident upon the reflecting surface 314 at
the same angle, they are reflected from the surface at two
different angles.
This bireflection problem occurs in the standard triangular three
sided ring resonator because two of the reflective surfaces are
non-symmetrical with respect to the crystal axes.
FIG. 3C shows the case where the reflecting surface 414 is parallel
to the crystallographic b-axis. In this case, an incident wave
polarized perpendicular to the a-b plane (represented by arrow 410)
is reflected in the direction of arrow 416. An incident wave
polarized parallel to the a-b plane (represented by arrow 412) is
reflected in the direction of arrow 418, which is now in the same
direction as arrow 412. Hence, both incident waves are reflected at
the same angle, regardless of their polarization. The lengths of
arrows 410 and 412 are from curves 202 and 200, respectively, to
the origin. The lengths of arrows 416 and 418 are from the origin
to curves 202 and 200, respectively.
These figures show that in order to have the same ring path 22 for
both orthogonally polarized modes 26 and 28 of FIG. 1, it is
necessary that mirrors 14, 16, 18, and 20 be disposed parallel to
axes of crystal symmetry. Our invention utilizes flat TIR mirrors
18 and 20 which are parallel to the crystal b-axis, and spherical
mirrors 14 and 16 having centers of curvature lying along the
crystal b-axis. The ring path 22 is symmetric about both the
crystal b-axis and the crystal a-axis.
While generation of blue-green light using KTP and GaAlAs and
strained-layer InGaAs diode lasers has been used in specific
embodiments of the present invention, generation of other
wavelengths using other nonlinear crystals and other lasers is
possible. Other nonlinear crystals such as lithium niobate, lithium
equilibrated lithium niobate, lithium potassium niobate, lithium
iodate, KTP, KTA, barium borate, LBO and periodically-poled KTP and
lithium niobate may have Type-II nonlinear interactions requiring
input beams of two orthogonal polarizations. The fundamental
wavelengths required for these interactions can be generated from
various laser systems, including GaAlAs laser diodes,
strained-layer InGaAs laser diodes, InGaAsP laser diodes, AlGaInP
laser diodes, single-frequency titanium:sapphire and dye laser
systems and other single frequency solid-state lasers such as
Nd:YAG lasers.
FIG. 4 shows a phase change optical data storage system 500 which
uses a laser system 502. Laser system 100 may be used for system
502. The light from system 502 is collimated by a lens 504 and
passes to a circularizing optical element 506. Element 506 emits
light having a circular cross-sectional beam pattern. Element 506
may be a prism.
The light then passes through a polarizing beam splitter 520 and a
quarter-wave plate 522. The light is reflected off of a mirror 524
and focused by a lens 526 onto an optical recording medium 530.
Medium 530 may be a phase change type of optical recording
medium.
The light reflected from medium 530 returns through lens 526, is
reflected off of mirror 524, passes through plate 522 to beam
splitter 520. Reflected light is then diverted by beam splitter 520
to an astigmatic lens 540. Lens 540 focuses the reflected light
onto an optical detector 542. The recorded spots of the medium 530
have different reflectivities and these differences are detected by
optical detector 542 as data one and zeros. Detector 542 also
provides focus and tracking signals.
FIG. 5 shows a magneto-optic data storage system 600 which uses a
laser system 602. Laser system 100 may be used for system 602. The
light from system 602 is collimated by a lens 604 and passes to a
circularizing optical element 606. Element 606 emits light having a
circular cross-sectional beam pattern. Element 606 may be a
prism.
The light then passes through leaky polarizing beam splitter 620.
Beam splitter 620 has reflectivities of Rp greater than zero and Rs
approximately equal to 1 (s and p represent the orthogonal
polarization components of the light). The light is then reflected
off of a mirror 624 to a lens 626 and is focused onto an optical
recording medium 630. Medium 630 may be a magneto-optic type of
optical recording medium.
The light reflected from medium 630 returns through lens 626,
reflects off of mirror 624, and enters beam splitter 620. Beam
splitter 620 diverts the reflected light to an amplitude beam
splitter 640. Reflected data light is diverted to a half-wave plate
642 and to a beam splitter 644. Reflected light of other amplitudes
passes straight through beam splitter 640. This light is focused by
an astigmatic lens 646 to a quad detector 648 to produce tracking
and focus signals.
The medium 630 has recorded spots having either an up or down
magnetic domain. The light reflected off of these spots have their
plane of polarization rotated one way or the other depending upon
the direction of the magnetic domain of the spot. Beam splitter 644
separates the reflected light depending upon which way the plane of
polarization has been rotated. The separated beams go to a lens 650
and an optical detector 652 or to a lens 660 and an optical
detector 662. The difference in output signals of output signals of
detectors 652 and 662 are the data ones and zeros. A more detailed
explanation of optical disk drive systems is given in
"Gradiant-Index Optics and Miniature Optics," SPIE, Vol. 935, p. 63
(1988) by Glenn T. Sincerbox.
The advantages of the present invention may now be understood. It
has been discovered that the efficiency of Type-II nonlinear
processes such as frequency doubling and sum-frequency mixing may
be enhanced using a monolithic crystal ring resonator in which four
mirrors define the path of light within the resonator. These
mirrors must be placed parallel to axes of crystal symmetry in
order to eliminate bireflection so that both polarizations required
for the nonlinear interaction traverse the same path within the
crystal. This invention is of particular use for generation of
blue/green light using a KTP crystal fashioned into a monolithic
resonator for second-harmonic generation and sum-frequency mixing
of GaAlAs and InGaAs laser diodes. An efficient output of
blue/green light is produced which is especially suited to optical
storage systems.
While the preferred embodiments of the present invention have been
illustrated in detail, it should be apparent that modifications and
adaptations to those embodiments may occur to one skilled in the
art without departing from the scope of the present invention as
set forth in the following claims.
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